Unequal Placental Sharing

Twin-to-twin transfusion syndrome (TTTS) is one of the most common conditions affecting monochorionic twins, identical twins who share one placenta. However, not all monochorionic twin pregnancies with unequal fluid levels or different-sized twins have TTTS. These characteristics can also be seen in a condition called unequal placental sharing.

When two fetuses share one placenta, their umbilical cords may implant anywhere — there is no set or predictable pattern. Depending on where they implant, one twin may get a smaller share of the placenta than the other, resulting in less blood flow and nutrition to one fetus, and more to the other. This is called unequal placental sharing.

As a result, the twins may grow differently. The "normal" twin typically has normal or generous, but not excessive, amniotic fluid level. The smaller twin may have either normal amniotic fluid volume or, if its growth becomes progressively restricted, can develop low fluid levels, called oligohydramnios.

Doctors use ultrasound to determine the severity of unequal placental sharing. The greater the difference in size and weight between the twins, and the lower the fluid levels for the smaller twin, the more serious the situation.

Doctors also monitor the blood flow in the twins' umbilical cords using Doppler ultrasound.

Unequal Placental Sharing Versus TTTS

It can be challenging to differentiate unequal placental sharing from TTTS. Many TTTS pregnancies have some element of unequal sharing, and many pregnancies with unequal sharing may have some element of TTTS. However, when unequal sharing is the more significant aspect of the problem, amniotic fluid discrepancies typically don't reach the levels seen with severe TTTS, and the issue is more about the size and weight discrepancy.

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There are no treatments for unequal placental sharing, because the amount of placental share for each twin cannot be changed. Amnioreduction and laser treatment are not beneficial, and could even make things worse, as the underlying problem is not transfusion between the twins.

Instead, the smaller twin is monitored very carefully for signs of difficulty. Depending on the stage of the pregnancy, this may lead to hospitalization for closer monitoring or delivery. It is very important to keep close watch on the smaller twin — if the smaller twin is stillborn, it can possibly cause serious problems for the surviving twin, due to the vascular connections between them.

In severe, early cases of unequal sharing, in which the smaller twin is severely growth restricted and at significant risk of dying before reaching a viable gestational age, radio-frequency ablation (RFA) of umbilical cord flow to that twin may be an option, in order to protect the normally grown twin from the possible adverse effects of the other twin's stillbirth.

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Reviewed by health care specialists at UCSF Benioff Children's Hospital.

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